Mechanical shutter has a speed limit, for instance 1/2000, 1/4000 or 1/8000. Electrical shutter can often go much faster (in terms of exposure; not in terms of readout speed, which can give rolling shutter). So for bright days, electrical shutter can replace the ND filters of old.
Also electrical shutters allow for electronic first curtain shutter, but I'm not entirely clear of the benefits beyond possibly reducing shutter shock.
From a camera building point of view, an electronic shutter is almost inevitable since the photons will have to start and stop being read out at some point, so you might as well enable those start and stop moments to be the beginning and end of the exposure. It just wasn't feasible to use only electronic shutters for stills images until recently, due to low readout speeds creating rolling shutter. Video cameras have much lower resolution, so also much lower rolling shutter